Process for the catalytic exhaust gas aftertreatment of engine combustion emissions

ABSTRACT

For the regeneration of an NO x  storage catalyst by means of hydrogen in a secondary hydrogen process, the hydrogen required for the regeneration is taken from a primary hydrogen process.

This application claims the priority of German Patent Document No. 10161 696.1, filed Dec. 15, 2001, the disclosure of which is expresslyincorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a process for the catalytic exhaust gasaftertreatment of engine combustion emissions by an NO_(x) storagecatalyst which can be regenerated by feeding hydrogen to the exhaust gasupstream of the NO_(x) storage catalyst.

In the case of lean-operated internal-combustion engines, particularlyin the case of direct-injection Otto engines, Diesel engines or hydrogenengines, which are operated at an overstoichiometric air-fuel ratio, itis necessary to reduce nitrogen oxides. In addition to continuouslyoperating systems, intermittently operating NO_(x) storage catalysts areused for this purpose which, in an overstoichiometric operation of theinternal-combustion engine, intermediately store NO_(x), which can bereleased again in shorter understoichiometric operating phases. Forexample, a platinum-containing adsorber is used which oxidizes NO byoxygen first to NO2. In the subsequent storage step, the acidic NO2 thenattacks a metal oxide and in the process forms the pertaining nitrate.In particular, alkali metals and alkaline earth metals (such as barium)can be used as storage material and, because of their basiccharacteristics, are capable of forming sufficiently stable nitrates inthe given temperature range. When the supply of storage oxides is usedup, the engine must be operated for a short time by a rich mixture inorder to regenerate the storage device. In this case, the reducingagents present in the rich mixture break up the nitrates and NO iscreated as well as the original metal oxide.

In the implementation of this concept, particularly the change from thelean to the rich operation required for the regeneration of the NO_(x)adsorbent is disadvantageous. In addition to the very high controlexpenditures, there are also the risk of HC and CO slip as well as aparticle/soot emission.

Remedies were provided by regenerating the NO_(x) storage catalyst byadding hydrogen to the untreated exhaust gas upstream of the NO_(x)storage catalyst. German Patent Document DE 199 39 807 A1 describes sucha process and a system for the exhaust gas aftertreatment particularlyfor lean-mix engines, such as direct-injection Diesel engines andgasoline engines, as well as maintaining the operability of NO_(x)storage catalysts in gasoline and diesel engines and particle filters indiesel engines. For the regeneration of the NO_(x) storage catalyst,hydrogen is intermittently added to the untreated exhaust gasapproximately once per minute in order to cause a reduction of thenitrogen oxides. According to German Patent Document DE 199 39 807 A1, ahigh-expenditure on-board hydrolysis unit is provided for obtaining thehydrogen, which hydrolysis unit, in addition to the actual electrolyzer,comprises a water reservoir, a metering device, a hydrogen reservoir forthe intermediate storage and a piping system.

This unit requires the costs of a separate hydrogen process thatincludes the process-related devices required exclusively for theregeneration of the NO_(x) storage catalyst. The hydrogen used for theregeneration of the NO_(x) storage catalyst has to be made availableonly for the purpose of regenerating the catalyst and, if required, hasto be intermediately stored in a hydrogen reservoir. The water reservoirhas to be refilled regularly.

It is therefore an object of the invention to provide a process whichovercomes the above-mentioned disadvantages.

According to the invention, this object is achieved by using a processwhich includes the withdrawal of the hydrogen used in a secondaryprocess for the regeneration of the NO_(x) storage catalyst from aprimary hydrogen process.

Within the scope of the invention, the term “hydrogen” also applies to ahydrogen-rich synthesis gas.

In the case of a motor vehicle, whose internal-combustion engine can beoperated with hydrogen, it is very expedient, if a partial quantity isextracted from the hydrogen flow fed to the internal-combustion engineand fed into the exhaust gas flow upstream of the NO_(x) storagecatalyst. Depending on the further design of the exhaust gasaftertreatment device, the feeding of the hydrogen to the exhaust gasflow takes place discretely or continuously.

In the case of a motor vehicle whose internal combustion engine isoperated with hydrocarbon fuels and at which motor vehicle the hydrogencontaining synthetic gas is produced in a primary hydrogen process by areformer, especially in order to operate a fuel cell system, it is veryadvantageous, if a partial quantity of this hydrogen-rich reformer gasis fed into the exhaust gas flow upstream of the NO_(x) storagecatalyst.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example of the regeneration of anNO_(x) storage catalyst comprising an adsorber part in the case of amotor vehicle whose internal-combustion engine can be operated withhydrogen;

FIG. 2 is a schematic view of an example of the regeneration of anNO_(x) storage catalyst comprising two adsorber parts arranged inparallel in the case of a motor vehicle whose internal-combustion enginecan be operated by hydrogen;

FIG. 3 is a schematic view of an example of the reaction processestaking place in a reformer system;

FIG. 4 is a schematic view of an example of the regeneration of anNO_(x) storage catalyst by hydrogen in the case of a motor vehicle whichcomprises a reformer; and

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 describes the regeneration of an NO_(x) storage catalyst for amotor vehicle whose internal-combustion engine can be operated withhydrogen in a primary hydrogen process. For the storage of the fuel, aninsulated tank 11 is provided in which cryogenic hydrogen is stored inthe liquid state at approximately −260° C. Through pipes 12, 12 c, thehydrogen reaches the internal-combustion engine 14 and is convertedthere by air fed through the pipe 13. The nitrogen oxides, which arecontained in the exhaust gas 15 of an overstoichiometric combustionduring lean mix operations, are fed to a No_(x) storage catalyst 18, andthe cleaned exhaust gas is led away by pine 19.

A portion of the hydrogen fed to the internal-combustion engine 14through the pipe 12 is extracted in a pipe 12 a and is fed in theexhaust gas flow upstream the NO_(x) storage catalyst 18 to a secondaryhydrogen process for the purpose of regenerating the storage device. Thehydrogen is stored in the tank 11 in the liquid state, while it reactsin the catalyst in the gaseous state. If the hydrogen is fed in a liquidstate through insulated pipes 12, 12 c to the internal-combustion engine14, it is advantageous for the extracted hydrogen to be evaporated inthe pipe 12 a, so that it is added in the gaseous state to the exhaustgas flow 15 upstream the catalyst 18. The evaporation of the hydrogencan be achieved in that the pipe 12 a is in a heat-exchanging contactwith another pipe carrying a warmer medium. The pipe 12 a extends, forexample, in a spiral shape around another pipe or is surrounded byanother pipe in a spiral shape. A further design with cooling ribs isalso expedient for warming up the hydrogen with air.

In the adsorption mode the NO_(x) storage catalyst cleans nitrogenoxides from the exhaust gas by first oxidizing NO by oxygen at theplatinum of the catalyst to NO2, and subsequently the acidic NO2corresponding to a storage adsorbs at metal oxide while forming thepertaining nitrate. With increasing degree of staturation of the NO_(x)storage catalyst, the availability of storage oxides is depleting sothat their regeneration is required. This is achieved by feedinghydrogen, and the hydrogen and the stored nitrogen oxides reactaccording to the following equation into nitrogen and water:H₂+1/xNO_(x ads)=>1/2×N₂+H₂O  (1)

In this case, the controlling of the hydrogen feed takes place such thata regeneration occurs as a function of the loading condition of thecatalyst. A high loading of the NO_(x) storage catalyst necessitates ahigh hydrogen addition to the untreated exhaust gas. When the loadingdecreases, the hydrogen feed is correspondingly discretely orcontinuously reduced and vice-versa.

FIG. 1 illustrates an embodiment in which the NO_(x) storage catalystcomprises an adsorber part 18 which, in different sections, can beoperated alternately in the adsorption or the desorption mode. For thispurpose, the hydrogen is fed, preferably continuously, to the exhaustgas flow upstream the NO_(x) storage catalyst 18 so that sections whichchange successively with respect to time are regenerated by hydrogen. Asa result of the treatment of all of these sections, the regeneration ofthe entire storage catalyst 18 takes place. For example, at a point intime t₀, a section 18 a is regenerated by hydrogen and, at a point intime (t₀+Δt), a section 18 c is regenerated by hydrogen. While thesections 18 b, 18 d continue to adsorb NO_(x) at the corresponding pointin time. For the admission of hydrogen to partial areas 18 a, 18 c ofthe cross-section 18, for example, a covering element is used which canbe rotated relative to the catalyst body and which divides the circularabsorber part 18 into sectors and, with which, the added hydrogen can bedirected to certain areas. According to another embodiment, theinjection nozzle for feeding the hydrogen can be moved so that atargeted supply takes place.

FIG. 2 shows an alternative further embodiment comprising an NO_(x)storage catalyst which has two adsorber parts 28 a, 28 b arranged inparallel. The exhaust gas flow, which contains nitrogen oxides and isdischarged through the pipe 25 from the internal-combustion engine 24,can be conducted through the pipes 25 a, 25 b through the adsorber part28 a and/or through the adsorber part 28 b; hydrogen can be fed throughthe pipes 22 a, 22 b to the exhaust gas flow upstream the catalyst 28.

As a result of the operations of the adsorber parts 28 a, 28 b whichchange successively with respect to time, in the adsorption ordesorption, mode a regeneration of one adsorber part can take place ineach case, while the other continues to be available for the cleaning ofthe exhaust gas. Thus, at the point in time t0, the adsorber part 28 bis regenerated by the feeding of hydrogen by way of the pipe 22 b, whichis illustrated as a solid line, to the untreated exhaust gas, while theadsorber part 28 a continues to adsorb nitrogen oxides from the exhaustgas fed by way of the pipe 25 a illustrated as solid line 25 a. At asubsequent point in time t₀+Δt, the adsorber part 28 a is regenerated byhydrogen by way of the pipe 22 a illustrated by a broken line so thatits regeneration takes place, while the adsorber part 28 b adsorbsnitrogen oxides from the exhaust gas. Subsequently, at a point in timet₀+2 Δt, both adsorber parts 28 a, 28 b are regenerated and are thenavailable for cleaning the exhaust gas.

Furthermore, it is very expedient to use the process according to theinvention in the case of a motor vehicle which, in a primary hydrogenprocess, comprises a fuel cell and which, for supplying the latter withhydrogen, comprises a reformer, as illustrated, for example, in FIG. 3.The internal-combustion engine of the motor vehicle is expedientlyoperated by hydrocarbon which is also available to the reformer forproducing a hydrogen-rich synthesis gas. Corresponding to the respectivetype of reforming, hydrocarbon-containing fuel 32 is reacted in thereformer 30 in a reaction zone 36 b by the addition of air 33 and/orwater 311 to a reformate including predominantly H2, CO and N2.

An emission control device of a motor vehicle of this type isillustrated in FIG. 4. A hydrocarbon-containing fuel, such as gasolineor Diesel fuel, is stored in a tank 41, which fuel is fed to aninternal-combustion engine 44 by way of pipes 42, 42 a. The exhaust gas,which is generated during the combustion with air fed by way of thepipes 43, 43 a and which contains nitrogen oxides in the lean-mixoperation, is fed to an NO_(x) storage catalyst 48 by way of the pipe 45for cleaning, which NO_(x) storage catalyst is regenerated in thatalternately sections 48 a, 48 c are treated by hydrogen, while sections48 b, 48 d continue to be available for the cleaning of the exhaust gas.The adsorption and desorption operations continue to correspond to thoseshown in FIG. 1.

The hydrogen required in a secondary hydrogen process for theregeneration of the NO_(x) storage catalyst 48 according to Equation (1)is partially extracted from the reformate provided to a fuel cell whichis not shown in this Figure. For producing the hydrogen,hydrocarbon-containing fuel is fed to the reformer 46 by way of the pipe42 b, as well as air and/or water is/are fed by way of the pipes 43 b,411, 412; by way of pipe 47, the reformate is added to the exhaust gasflow upstream the NO_(x) storage catalyst 48 for the purpose ofregeneration. The cleaned exhaust gas is led away by way of a pipe 49,49 b, in which case it is very expedient to conduct at least a partialflow by way of a pipe 49 a, for the purpose of recovering water througha condenser 49 c. Also in this embodiment, naturally—as illustrated anddescribed by FIG. 2—an NO_(x) storage catalyst 48 can be used which hastwo adsorber parts.

1. A process for catalytic exhaust gas aftertreatment of enginecombustion emissions with an NO_(x) storage catalyst which can beregenerated by feeding hydrogen to the exhaust gas upstream of theNO_(x) storage catalyst, the process comprising: feeding the hydrogenused in a secondary process for the regeneration and desorption of theNO_(x) storage catalyst or adsorber catalyst from a primary hydrogenprocess; in the case of a motor vehicle whose internal-combustion engineis operated in a primary hydrogen process with only hydrogen, extractinga partial quantity of hydrogen from the hydrogen flow supplied to theinternal-combustion engine and feeding the partial quantity to theexhaust gas flow upstream of the NO_(x) storage catalyst; and downstreamof the NO_(x) storage catalyst, condensing water vapor contained in theexhaust gas and feeding it to a hydrogen process.
 2. A process forcatalytic exhaust gas aftertreatment of engine combustion emissions withan NO_(x) storage catalyst which can be regenerated by feeding hydrogento the exhaust gas upstream of the NO_(x) storage catalyst, the processcomprising: feeding the hydrogen used in a secondary process for theregeneration and desorption of the NO_(x) storage catalyst or adsorbercatalyst from a primary hydrogen process; in the case of a motor vehiclewhose internal-combustion engine is operated in a primary hydrogenprocess with only hydrogen, extracting a partial quantity of hydrogenfrom the hydrogen flow supplied to the internal-combustion engine andfeeding the partial quantity to the exhaust gas flow upstream of theNO_(x) storage catalyst; and regenerating different sections of anadsorber material of the NO_(x) storage catalyst in a successive mannerwith respect to time.
 3. An engine exhaust system comprising: an NO_(x)storage catalyst which is regenerable by feeding hydrogen to the exhaustgas upstream of the NO_(x) storage catalyst, wherein hydrogen is fedfrom a primary hydrogen process to a secondary process for theregeneration and desorption of the NO_(x) storage catalyst or adsorbercatalyst, wherein, in the case of a motor vehicle whoseinternal-combustion engine is operated in a primary hydrogen processwith only hydrogen, a partial quantity of hydrogen is extracted from thehydrogen flow supplied to the internal-combustion engine and fed to theexhaust gas flow upstream of the NO_(x) storage catalyst, wherein,downstream of the NO_(x) storage catalyst, water vapor contained in theexhaust gas is condensed and fed to a hydrogen production process.
 4. Anengine exhaust system comprising: an NO_(x) storage catalyst which isregenerable by feeding hydrogen to the exhaust gas upstream of theNO_(x) storage catalyst, wherein hydrogen is fed from a primary hydrogenprocess to a secondary process for the regeneration and desorption ofthe NO_(x) storage catalyst or adsorber catalyst, wherein, in the caseof a motor vehicle whose internal-combustion engine is operated in aprimary hydrogen process with only hydrogen, a partial quantity ofhydrogen is extracted from the hydrogen flow supplied to theinternal-combustion engine and fed to the exhaust gas flow upstream ofthe NO_(x) storage catalyst, wherein the NO_(x) storage catalystincludes an adsorber material being regenerated section-wise in asuccessive manner with respect to time.